Internal burners operated on compressed air and fuel have not been used in the commercial flame spray field. Present flame spray systems use expensive pure oxygen or even electricity to create a flame spray jet stream for accelerating particle movement while melting the particles prior to impact on a substrate downstream of the flame spray apparatus.
Internal burners for use with compressed air are cumbersome and are large in size, and the exit flame-jet from a single outlet nozzle cannot be readily placed coaxially with injected powder or solid wire or rod material to be flame sprayed by the apparatus.
My U.S. Pat. No. 4,416,421 is directed to a flame spray method and apparatus wherein pure oxygen and a fuel mixture continuously flowing to and confined within an essentially closed internal burner combustion chamber creates relatively high pressure hot combustion product gases which are directed through a flow expansion nozzle as a high velocity gas stream at one end of the combustion chamber. Material in solid rod form or as solid particles is fed to the hot gas stream outside of the combustion chamber and axially into a converging flow of the hot combustion product gases, after exit from the combustion chamber, and while entering a converging portion of a flow expansion nozzle whose length is at least five times that of the nozzle bore throat. Thus, the material can be heat softened or liquified and sprayed at high velocity onto a surface positioned in the path of the stream at the discharge end of the nozzle. This results in a restriction of the diameter of the column of particles passing to the nozzle bore, the prevention of build up of particle material on the nozzle bore wall while insuring sufficient particle dwell time within the bore to effect particle heat softening or melting and flow at supersonic flow velocity prior to impact against the surface.
The present invention further provides an improvement over my copending U.S. patent application Ser. No. 571,775 filed Jan. 18, 1984. In application Ser. No. 571,775, the material to be flame sprayed is introduced in a supersonic flow stream prior to passing with the stream through a nozzle of extended length. In such application, flow pressures are always greater than atmosphere at the point of introduction of the solid material in either powder or rod form to the reactant products of combustion and the reactants constitute oxygen and fuel continuously supplied to the internal burner.
Introducing the solid material in powder or rod form into the hot flowing product of combustion gases at a zone of sub-atmospheric gas pressure has distinct advantages. Reactants of compressed air and fuel permit the successful melting of lower melting point materials such as aluminum, zinc, copper, and flame spraying of these materials at substantially reduced cost relative to the oxy-fuel mixture which is necessary when melting and spraying higher melting point materials such as stainless steel, mild steel, nickel, chrome alloys, and the like.
In flame spray systems, in order to prevent build up of the material being flame sprayed, after melting or after being rendered semi-molten, conventionally such material in either rod or powder form is introduced axially into the hot products of combustion gas stream which in itself is delivered to the nozzle under conditions of flow symmetry, as in U.S. patent application Ser. No. 571,775, thereby tending to maintain the particles within the core of the gas stream, while accelerating the stream of particles to supersonic velocity prior to impact against a substrate downstream from the exit of the nozzle itself.
It is a primary object of the present invention, in one form, to provide an improved compressed air and fuel flame spray method and apparatus which permits compressed air to be employed as an element of the fuel mixture within an internal burner, which effectively permits large scale flame spraying of lower melting point materials with huge cost savings by eliminating the necessity to employ expensive pure oxygen and which results in high material deposition rates, produces dense coatings and which inherently eliminates back flow of the products of combustion into the material feed mechanism.
It is a further object of the present invention to provide an improved flame spray method and apparatus of the internal burner type in which either a continously flowing compressed air and fuel mixture or a pure oxygen and fuel mixture is ignited within a combustion chamber, and wherein solid material in powder or rod form is introduced into the products of combustion, outside of the internal burner combustion chamber, at sub-atmospheric pressure after the gas overexpanded, with either the gas or the material, if in powder form, introduced symmetrically or asymmetrically without particle build up along the nozzle wall.